Fuel tanks can contain potentially combustible combinations of oxygen, fuel vapors, and ignition sources. In order to prevent combustion in aircraft fuel tanks, aviation regulations require actively managing the ullage of fuel tanks, such that the oxygen partial pressure in the ullage is less than 12%. Relatedly, fire suppression systems, such as those deployed in aircraft cargo holds, use halogenated chemicals to prevent combustion and/or fire. Halogenated fire suppression agents can be safe for human exposure; however, they are known to be detrimental to the Earth's atmospheric ozone layer. Inert air can be used for fire prevention and suppression.
Currently, many On-Board Inert Gas Generation Systems (OBIGGS) use bleed air and pressurized hollow fiber membranes to produce inert gas for fuel tank ullages. In hollow fiber membranes, the diffusivity of nitrogen is less than the diffusivity of oxygen and water vapor. Hollow fiber membrane systems require pressurized air to drive the separation of nitrogen from oxygen and water vapor in an air stream. However, the pressure of bleed air extracted from an aircraft engine compressor varies throughout a mission, which affects inert gas production quantity and quality as defined by oxygen partial pressure. Furthermore, aircraft design is trending toward lower pressure bleed systems and increasingly electric power distribution architectures. Accordingly, the use of high pressure, hollow fiber membrane inerting systems can be problematic for these systems.
Other approaches utilize catalytic reactors to produce inert gas from ullage space fuel vapors, or from liquid fuel. The ullage space, however, may not always contain a sufficient amount of fuel vapors to provide for reaction. Thus, a system capable of maintaining a safe oxygen partial pressure in the ullage is necessary in order to comply with regulations requiring ullage passivation throughout the mission.